1. Field of the Invention
The subject matter of this invention relates to a process for removing the soluble metal cations found in opaque or transparent iron oxide pigments. The resulting pigments can be used as colorants in substances which are ingested by human beings and animals, and they can also be used to stabilize plastics against ultraviolet degradation.
2. Description of the Prior Art
Iron oxide pigments are found in nature as mineral deposits. There are three types of iron oxide pigments which are found in nature as mineral deposits. These are limonite, hematite, and magnetite. Limonite is yellow and has a chemical formula of Fe.sub.2 O.sub.3 .multidot.xH.sub.2 O where x is an integer. As can be seen from its formula, it is a hydrated iron(III) oxide. Hematite is red and has a chemical formula of Fe.sub.2 O.sub.3. It is an anhydrous iron(III) oxide. Magnetite is black and has a chemical formula of Fe.sub.3 O.sub.4. It is considered to be a spinel containing iron(II) and iron(III) ions.
In addition to the natural mineral deposits, iron oxide pigments can be obtained synthetically. Synthetic methods for preparing iron oxide pigments generally involve the alkali precipitation of iron(II) compounds from a soluble iron(II) salt and the oxidation of the precipitated iron(II) compound to an iron(III) oxide pigment slurry. The pigment produced by such methods corresponds to the yellow hydrated iron(III) oxide described previously. The yellow pigment is recovered from the slurry by filtration, washing, and drying. The alkali precipitation must occur under acidic conditions; otherwise, dark brown or black undesirable color shades will be produced. Iron oxide pigments of various yellow shades can be prepared by controlling the temperature and rate of oxidation. Red, black and other colors can be prepared by calcining the yellow pigments at high temperatures.
Iron oxide pigments which have an average particle size less than 0.1 micron are considered to be transparent because they can transmit visible light. Iron oxide pigments which have an average particle size greater than 0.1 micron and which cannot transmit visible light are considered to be opaque. Generally, naturally occuring iron oxide pigments are opaque.
Whether the iron oxide pigments are mined or manufactured, transparent or opaque, they have high concentrations of soluble metal cations such as antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, manganese, mercury, nickel, selenium, thorium, tin, and zinc. These soluble metal cations may be present in varying amounts, but in high quality pigments generally do not exceed quantities such that the pigment conductivity as measured by ASTM D-2448-73 exceeds 3000 .mu.mho.
These soluble metal cations must be distinguished from bulk, insoluble salts. Bulk insoluble salts, such as calcium sulfate and calcium carbonate scales do not generally occur in the preparation of high quality iron oxide pigments. Moreover, when such impurities do occur, they are readily removed by physical methods such as that disclosed in U.S. Pat. No. 2,904,402, or by washing with dilute acid. These treatments, however, are ineffectual in removing soluble metal cations, which are adsorbed onto the surface of the pigment itself.
Due to the fact that these soluble cations are adsorbed onto the pigment surface, the problem is especially acute with regard to the transparent iron oxide pigments, which, due to their small particle size, have enormous surface area. Previous attempts to remove the soluble cations, for example by conventional water washing has produced pigments with conductivities, as measured by ASTM D-2448-73, of from 1500-2000 .mu.mho's. Extended washing does not serve to lower the conductivity appreciably.
The presence of soluble metal cations greatly restricts the use of iron oxide pigments. They cannot be used as colorants in substances ingested by man and animals, such as food and medicine, because the soluble metal cations will catalyze oxidative reactions which will cause the substances to spoil.
The soluble metal cations will also cause plastics and coatings to degrade. Consequently, iron oxide pigments, which act as stabilizers against ultraviolet radiation of wavelengths between 300 nanometers and 400 nanometers, cannot be used for these purposes, or for use in some of the newer coating systems such as the base-coat/clear-coat. In this process, for example, an initial base-coat of paint containing the pigment is applied, following which a transparent, or clear-coat is applied. This process has the advantage that the pigment is protected against oxidation, and moreover, that minor surface scratches do not remove pigment and are thus more susceptible to successful repair efforts. Unfortunately, if the pigment has any significant level of soluble metal cation impurities, these impurities migrate out of the pigment coat into the interface between the pigment-containing coat and clear-coat, causing the latter to separate. Generally, pigments for this application must have soluble cation impurity levels such that conductivity tests of the leachate,as measured by ASTM D-2448-73 exhibit a conductivity less than about 500 .mu.mho to be successful, while values of 400 mmho's or less are preferred.
Accordingly, there is a need to develop a method to deactivate or remove the metal cations present in iron oxide pigments in order to expand the application of these pigments. This is particularly so since the iron oxide pigments are suspected of being nontoxic and non-carcinogenic, and could be substituted for many of the organic pigments and dyes which are believed to be toxic and/or carcinogenic.